Frost detector for refrigeration system

ABSTRACT

A frost detector for an air-cooling refrigeration system having an evaporator coil mounted in a main airflow duct is comprised of a first bypass duct connected to the main duct on each side of the coil and a second reference bypass duct connected to each side of a constriction in the outlet side of the main duct downstream from the coil. Signals from airflow velocity sensing devices in each bypass duct are compared to provide a defrost initiation signal to the refrigeration apparatus and the preferred airflow velocity sensing device is an air-cooled electrical resistor connected in a bridge circuit.

Edited Wrinkles Feb. 22, 1W7? [54] IFRQST DE'lFlEQTUR lF UR 3,377,817 4/1968 Petranek ..62/140 REFRKGERATMUN YST1EM 3,423,222 911369 Sutton, Jr. et a1 ....62/l51 3,4 l l 70 Lorenz ..62/140 [72] Inventor: Charles IL. Winkler, Pittsburgh, Pa. [73] Assignee: Westinghouse Electric (Importation, Pitt- Primary Emmif'er Wmiam ODea Sburgh, Pa Assistant Exammer--P. D. Ferguson Attorney-F. H. Henson and F. E. Blalce [22] Filed: Nov. 20, 1970 21 Appl. No.: 91,299 [57] Amsmm A frost detector for an air-cooling refrigeration system having an evaporator coil mounted in a main airflow duct is coma! "62/13 prised of a first yp duct connected to the main duct on each side of the coil and a second reference bypass duct con- [58] Freid oi Search ..62/140, 151, 340/234 nected to each Side of a constriction in the Outlet side of the [56] References Cited main duct downstream from the coil. Signals from airflow velocity sensing devices in each bypass duct are compared to UNITED STATES PATENTS provide a defrost initiation signal to the refrigeration apparatus and the preferred airflow velocity sensing device is an lg; g p f air-cooled electrical resistor connected in a bridge circuit. utton, r. et a. 3,362,183 1/1968 Sutton, Jr. ..62/1 40 41 Claims, 2 Drawing Figures PAIENIEUHB 2 2 4972 3,, 6 3 5 Y HG. l

v |NV ENTOR WITNESSES C. L. Wmkler may on game) ATTORNEY BACKGROUND OF THE INVENTION Frost detection systems for refrigeration and air conditioning systems of the type having evaporator coils in a main airflow duct to detect the buildup and accumulation of frost on the evaporator coil to thereby provide a defrost signal to initiate a defrosting of the coil are well known. One of the most common frost detector systems uses a bypass duct connected to the main airflow duct on each side of the evaporator coil in the main duct so that when frost builds up on the coil, an increased airflow velocity occurs in the bypass duct and a suitably calibrated airflow velocity sensor provides the defrost signal when the airflow velocity in the bypass duct is such as to indicate an undesirable amount of frost on the coil. A difficulty with the aforementioned frost detection system is that the fixed calibrated airflow sensing device could provide a defrost signal when there are changes in airflow velocity that are not due to frost accumulation on the coil such as for example a deliberate change in airflow velocity by changing the blower speed or the like.

PRIOR ART Reference may be made to the U.S. Pat. No. 3,355,904, to Sutton, Jr. et al., issued Dec. 5, 1967, which discloses a refrigeration system frost detector using differential fluid velocity sensing in an arrangement which however does not provide the particular reference bypass duct arrangement of this invention which is located across a constriction of the outlet duct in a manner to assure proper tracking of the variable airflow velocity sensing devices over all of the possible variable conditions of operation including changed blower speeds, randomness of frost buildup on the coil, and changing temperature ambients or the like.

SUMMARY In accordance with the invention, the refrigeration system evaporator coil is positioned in a main airflow duct having a blower in its inlet side and a constriction in its outlet side downstream from the coil. A first bypass duct is connected to the main duct on each side of the coil and a second or reference bypass duct is connected to the main duct on each side of the constriction in the outlet. The airflow velocity in the first bypass duct will increase with the accumulation of frost on the coil assuming a particular fixed blower capacity but will also change with changes in blower speed. The airflow velocity through the second bypass duct around the constriction in the outlet of the main duct will be proportional to the total mass flow rate and therefore may increase or decrease with corresponding changes in blower speed and the like but will not increase with buildup of frost and on the contrary, will decrease with buildup of frost. By comparing the airflow velocity in both bypass ducts a defrost signal can be obtained that will accurately correspond to the given amount of frost buildup on the coil irrespective of the randomness of the frost buildup or of changes in blower speed or the like. In the preferred form of this invention, the airflow velocity sensor in each bypass duct is an air-cooled electrical resistor connected in a resistance bridge circuit. In order to maximize the sensitivity of the air-cooled sensing resistors in the system, the range of air velocities through the bypass ducts may be adjusted by throttle valves if desired.

Other advantages and features of the invention will be apparent with reference to the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FlG. l is a sectional elevation of an air-cooled refrigeration system having the frost detecting arrangement of the invention; and

FIG. 2 is a schematic of the bridge circuit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawings, the evaporator coil lltl is shown to be positioned within a main airflow duct 11 having a blower fan 12 in the inlet side thereof and a constructed outlet side 13. A first bypass duct 15 is connected to the main duct 11 on each side of the evaporator coil as shown at 16 and 17 respectively. An airflow velocity sensing device lltl is located within the first bypass duct 15 and a throttle valve 19 may be provided to control the range of air velocities through the bypass duct 15 if desired. A second bypass duct 20 which is a reference bypass duct is connected across the constriction in the outlet side of the main duct lllr It will be noted that the constriction for the main duct 11 is formed by the converging duct wall 25 to which one end 26 of the reference bypass duct 20 is connected. The other end 27 of the reference bypass duct 20 is connected to the constricted outlet side 12 of the main duct 11. An airflow velocity sensing device 23 is located within the reference bypass duct 20 and if desired a throttling valve 24 may also be provided to control the range of airflow velocities passing through the reference bypass duct 20.

For a given speed of the blower fan 12, the airflow velocity in the first bypass duct l5 will increase when frost builds up on the evaporator coil It) to impede or block the flow of air from the blower 112 through the main duct 11. Of course any change in the main flow velocity due to a change in blower fan speed 12 will also change the flow velocity in the first bypass duct 15. Therefore if the flow-rate sensor 18 is calibrated to provide a signal for a particular flow velocity, a false defrost signal could be obtained when the flow velocity increases in the bypass duct 15 due to an increase in speed for the blower fan 12. To compensate for any change in the main flow rate as provided by the speed of the blower 12, a reference bypass duct 20 is mounted across the constriction in the outlet 13 of the main airflow duct 11. The static pressure drop across the constriction varies approximately as the square law of the total mass airflow velocity, as it does across a well-known Venturi flow meter. The reference duct flow velocity, being a square root law of the static pressure drop, will therefore be proportional to the total mass flow velocity. By comparing the two signals from the airflow velocity sensors 18 and 23 in the bypass ducts 1S and 20, respectively, a defrost signal can be obtained which will indicate or detect the buildup of frost on the evaporator coil 10 regardless of changes in main airflow provided by the blower fan 12 and regardless of the randomness ofthe frost buildup on the coil 10.

Although the invention in its broadest sense is not limited to a particular type of airflow velocity sensor to be used in the bypass ducts 15 and 20, the preferred form of the invention employs air-cooled temperature sensitive resistors in a re sistance bridge circuit.

Referring to FIG. 2 of the drawing, the airflow temperature sensitive resistor 18 in the first bypass duct 15 is connected in a resistance bridge circuit with the adjustable resistor 30, fixed resistor 31 and the airflow sensing temperature sensitive resistor 23 in the reference bypass duct 20. Each of the temperature sensitive resistors 18 and 23 may preferably have a posi tive temperature coefficient although the invention is not limited to the use of such resistors since it would be obvious to anyone skilled in the art that other forms of temperature sensitive resistors such as those having; negative temperature characteristics may also be used. A direct current power supply 33 is connected to the input terminals of the resistance bridge circuit and a defrost signal can be obtained by an unbalance of the bridge across the output terminals 34 and 25. The adjustable resistor 30 is adjusted to balance the bridge for no output signal when the airflow rates sensed by the sensing resistors 18 and 23 are of the order corresponding to a condition of less than the undesired amount of frost on the evaporator coil 10. When the frost has built up on the evaporator coil 10 to an undesired amount the increase in flow velocity through the first bypass duct 15 relative to the flow velocity sensed by the reference bypass duct 20 would change the resistance of the sensing resistor 18 relative to the sensing resistor 23 such as to unbalance the bridge and provide a defrost initiation signal across the bridge output terminals 34 and 35. it is known that some temperature sensing resistors would have different sensitivities for different velocities of airflow across the resistors to cool the same. In order to maximize the sensitivity of the flow-rate temperature sensing resistors 18 and 23, the throttle valves 19 and 24 may be provided in the respective bypass ducts l5 and 20. It should now be obvious to anyone skilled in the art that by suitably adjusting the throttle valves 19 and 24 and by suitably adjusting the bridge balancing resistor 30 a desired sensitivity and range of operation for the frost detector of the invention will be obtained.

It will be noted that each of the bypass ducts l5 and have cross-sectional areas considerably less than the cross-sectional area of the main duct 11 including the cross-sectional area of the constricted outlet 13 of the main duct 11. This is of course desirable in order to prevent either bypass duct from effectively shunting the evaporator coil or the system and is also desirable in improving the sensitivity of the system. There has been no description of the apparatus for utilizing the defrost signal generated across the bridge output terminals 34, 35 since any well-known defrosting apparatus may be used and such apparatus is not a part of the present invention. Although the reference bypass duct 20 is shown to have one end 21 connected along the converging constriction wall 25 of the duct 11, it should be understood that the end 21 of the reference bypass duct 20 may be connected at any point along the main duct 11 upstream from the constricted outlet 13.

Various modifications will occur to those skilled in the art.

I claim:

1. A frost detector for an air-cooling refrigeration system comprising, a main airflow duct, an evaporator coil substantially enclosed in said duct, means to force air to be cooled through said duct and coil from an inlet side to an outlet side of the duct, the outlet side of said duct having a constriction to reduce the cross-sectional area of the duct, a first bypass airflow conduit connected to said duct on each side of said coil ahead of the constriction of the duct, a second bypass reference airflow conduit connected to the outlet side of said duct between spaced points along the constriction of the duct, said first and second bypass conduits having cross-sectional areas considerably less than any cross-sectional areas of the duct, a respective airflow velocity sensor in each of said first and second bypass conduits, and means to compare the airflow velocity sensed by each sensor in the respective bypass conduits so that a change in airflow velocity through said coil due to the accumulation of frost on said coil may be detected regardless of variations in operation of the means to force air through the duct and coil and the randomness of the accumulation of frost on the coil.

2. The invention of claim 1 in which each sensor is comprised of an air-cooled electrical resistor, and each sensor resistor is connected in a resistance bridge circuit to compare their changes in resistance due to changes in airflow through the respective bypass flow sensing conduits.

3. The invention of claim 1 in which each of said bypass conduits is provided with a variably controlled airflow throttle to facilitate the comparison of sensed changes in airflow through the respective bypass conduits.

4. The invention of claim 3 in which each sensor is comprised of an air-cooled electrical resistor, and each sensor resistor is connected in a resistance bridge circuit to compare their changes in resistance due to changes in airflow velocities through the respective bypass conduits. 

1. A frost detector for an air-cooling refrigeration system comprising, a main airflow duct, an evaporator coil substantially enclosed in said duct, means to force air to be cooled through said duct and coil from an inlet side to an outlet side of the duct, the outlet side of said duct having a constriction to reduce the cross-sectional area of the duct, a first bypass airflow conduit connected to said duct on each side of said coil ahead of the constriction of the duct, a second bypass reference airflow conduit connected to the outlet side of said duct between spaced points along the constriction of the duct, said first and second bypass conduits having cross-sectional areas considerably less than any cross-sectional areas of the duct, a respective airflow velocity sensor in each of said first and second Bypass conduits, and means to compare the airflow velocity sensed by each sensor in the respective bypass conduits so that a change in airflow velocity through said coil due to the accumulation of frost on said coil may be detected regardless of variations in operation of the means to force air through the duct and coil and the randomness of the accumulation of frost on the coil.
 2. The invention of claim 1 in which each sensor is comprised of an air-cooled electrical resistor, and each sensor resistor is connected in a resistance bridge circuit to compare their changes in resistance due to changes in airflow through the respective bypass flow sensing conduits.
 3. The invention of claim 1 in which each of said bypass conduits is provided with a variably controlled airflow throttle to facilitate the comparison of sensed changes in airflow through the respective bypass conduits.
 4. The invention of claim 3 in which each sensor is comprised of an air-cooled electrical resistor, and each sensor resistor is connected in a resistance bridge circuit to compare their changes in resistance due to changes in airflow velocities through the respective bypass conduits. 